Amphibia (redirected from class Amphibia)

Amphibia

One of the four classes composing the superclass Tetrapoda of the subphylum Vertebrata, the other classes being Reptilia, Aves, and Mammalia. The living amphibians number approximately 2460 species, and are classified in three orders: the Anura or Salientia (frogs and toads, slightly less than 2000 species); Urodela or Caudata (salamanders, 300 species); and Apoda or Gymnophiona (caecilians, about 160 species). The orders in the subclasses Labyrinthodontia and Lepospondyli existed in the geologic past and are now extinct. A classification scheme for the Amphibia follows:

• Class Amphibia
• • Subclass Labyrinthodontia
  • • Order: Ichthyostegalia
    • Temnospondyli
    • Anthracosauria
  • Subclass Lepospondyli
  • • Order: Nectridea
    • Aistopoda
    • Microsauria
    • Lysorophia
  • Subclass Lissamphibia
  • • Order: Anura
    • Urodela
    • Apoda

A typical amphibian is characterized by a moist, glandular skin, the possession of gills at some point in its life history, four limbs, and an egg lacking the embryonic membrane called the amnion. See Amnion, Anamnia

The closest relatives of the amphibians are the fishes, from which they evolved, and the reptiles, to which they gave rise. Present-day amphibians, however, are highly specialized animals, rather different from the primitive forms that probably first arose from crossopterygian fishes and far removed from those that gave rise to the earliest reptiles.

In general, modern amphibians as adults differ from fishes in lacking scales, breathing by means of the skin and lungs instead of gills, and having limbs in place of fins. There are many exceptions to these generalizations, however. Reptiles usually have a dry, scaly skin that is relatively impervious to water loss and very different from the amphibians with their moist skin that permits much evaporation. Young (larval) amphibians have gills, but there is no comparable gill-breathing, larval stage in the life history of a reptile. A most important difference between the two groups is the absence of the amnion in the Amphibia, and its presence in the Reptilia. Lacking this membrane, amphibian eggs must be laid in water or in very moist places. The amnion of the reptile egg makes it more able to resist desiccation, and the eggs can be laid in relatively dry places. The ability to resist water loss through the skin and the development of a land egg are perhaps the differences between reptiles and amphibians that are of the greatest evolutionary significance. See Reptilia

The all-important factor in amphibian life is water. Most species must return to the water to breed, and all must have access to water (even if only in the form of rain or dew) or die of dehydration in a short time. An important consequence of this basic fact of physiology is that vast arid and semiarid areas of the Earth are inhabited by a relatively few specialized amphibians. The majority of amphibian species are found in moist, tropical regions.

Amphibians are among the so-called cold-blooded animals; that is, the temperature of the body of an amphibian is not regulated internally to a high level as is that of mammals and birds, but fluctuates with that of the environment. An animal such as an amphibian that burns none of its food energy in keeping warm is able to get along on much less food than a bird or mammal of similar size. This advantage is offset by the inability of amphibians to be active under cold conditions that do not inhibit a warm-blooded animal. Thus the far northern and southern parts of the world which support large populations of birds and mammals are almost devoid of amphibian life. The amphibians mark a significant point in the evolution of the vertebrates, the transition from aquatic to terrestrial life. As animals neither divorced from the water nor fully at home on land, they suffer from their intermediate mode of life. Reptiles, and later mammals, came to dominate the land, and fishes the waters, leaving the amphibians of today as a relatively unimportant but nevertheless highly interesting group of vertebrates. See Thermoregulation

The fossil record of the three groups of living amphibians is extensive, and the earliest member of each has been found in Mesozoic rocks. However, no intermediary forms linking the three groups together have been found in the Mesozoic, and it is necessary to look in the Paleozoic, some 100 million years earlier, for the common ancestor of modern amphibians, with the earliest known amphibians having been found in the Upper Devonian rocks of Greenland.

It is clear that modern amphibians have a very long history extending back almost to the time of the origin and radiation of land vertebrates 340 million years ago. Their unique sensory biology and specialized glands must have evolved at that time and remained unchanged to the present day.